This invention relates to a direct current power supply for manual arc welding a steeply drooping, adjustable characteristic. In this context, the expression "steeply drooping" should be understood to indicate that the short circuit current exceeds the arc current by not more than 50%. More particularly, the invention relates to a power supply of the type comprising a polyphase transformer, a polyphase rectifier system connected to the secondary of said polyphase transformer, said rectifier system comprising a set of triggered rectifier valves connected one in series with each secondary phase circuit of the polyphase transformer, a pair of welding current conductors connected to the output of said polyphase rectifier system, means for controlling the firing angle of the triggered rectifier system in response to a direct current control signal, and signal generating means for producing said direct current control signal comprising
Means for producing an adjustable direct current reference signal, PA1 Means including a welding current sensing means for producing a welding current feed-back direct current signal, and, PA1 Summing means for combining said reference signal and said welding current feed-back signal into a compound signal constituting the control signal aforementioned, said feedback signal being supplied to said summing means in opposing relationship to said reference signal. PA1 means including a welding current sensing means for producing a welding current feed-back direct current signal bearing a substantially fixed proportion to the welding current, PA1 means including a welding voltage sensing means for producing a welding voltage feed-back direct current signal proportional to the welding voltage, PA1 summing means for combining said reference signal, said welding current feed-back signal and said welding voltage feed-back signal into a compound signal constituting the control signal aforementioned, both of said feed-back signals being supplied to said summing means in opposing relationship to said reference signal, PA1 said means for producing a welding voltage feed-back signal including means for adjustment of the slope of the power supply characteristic through adjustment of the factor of proportionality between said voltage feed-fack signal and the welding voltage.
In a welding power supply of this kind described in the U.S. Pat. No. 3,530,359, the adjustable D.C. reference signal is obtained by the combining of two component signals, both of which are individually adjustable. More particularly, one adjusting member is provided for the first component, while two adjusting members independent of each other and of said first-mentioned adjusting member are provided for the second component. One of said two last-mentioned adjusting members also serves to control the slope of the characteristic of the power supply through adjustment of the factor of proportionality between the welding current feed-back signal and the welding current. Consequently, the proper adjustment of the apparatus involves the manipulation of three independent controls, which is apt to confuse the welder. Also, the welding current delivered by a power supply of this kind varies strongly with the line voltage. The disturbing action of line voltage fluctuations increases with the forward slope of the characteristic. To minimize the disturbing action of line voltage fluctuations the slope control has to be set at as steep a slope as possible, that is, a slope approaching the vertical as nearly as possible. On the other hand, it is a well-known fact that, in welding with coated electrodes, the short circuit current must exceed the arc current by not too small an amount in order to ensure an orderly metal transfer from the electrode to the workpiece and to prevent "stubbing" and other undesirable effects. With the control system referred to it is, therefore, not possible to combine good welding characteristics with the desired insensitivity to line voltage fluctuations.